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Actions to limit climate change and calls for water conservation are on a potential collision course, some water researchers believe.

Making low-carbon electricity from sources like concentrated solar, geothermal and coal with carbon capture and sequestration consumes more water than fossil fuel-generated power, experts said. Developing cleaner-burning motor fuels also requires more water than turning oil into gasoline.

With severe droughts in California, Colorado, New Mexico, Nevada and other states, water consumption is becoming a mounting concern.

"It is a balance that a lot of utilities in the Southeast and the Southwest are really looking at closely," said Michael Hightower, Water for Energy project lead at Sandia National Laboratories. It's also becoming an issue in parts of the Midwest and Northeast, he said. For power plants looking to do carbon capture and storage, "they're trying to figure out where they're going to get the water."

People think of global warming as increased temperatures, Hightower said, but it's also about changes in precipitation and water supply. While there's been an emphasis on carbon footprints, he said, "the water footprint is going to be just as important."

Making 1 megawatt-hour of power at a fossil fuel-powered plant on average nationwide uses 1,200 to 2,000 liters of water, according to data Hightower submitted to Congress. A nuclear plant consumes 1,600 to 2,900 liters for that megawatt-hour. Concentrated solar generation -- which uses mirrors instead of photovoltaic panels -- can drink up 2,900 liters. A geothermal steam plant can use as much as 5,500 liters.

"The electricity sector is the largest withdrawer of freshwater in the nation," a 2012 analysis from the National Renewable Energy Laboratory said.

The nexus is expanding in importance because of droughts and as utilities replace aging power plants. An EPA rule is forcing power plants to examine water use. Many are switching away from open-looped cooling, which pulls water in from a nearby river or ocean, uses it for cooling, then discharges it at a higher temperature. Closed loop -- using water that is cooled in a tower -- is preferred for environmental reasons but can be more costly and can require double the amount of on-site water.

Water consumption also is a focus as states seek to cut carbon emissions. California has the country's most ambitious goal, aiming to shrink that pollution to 1990 levels within six years and 80 percent below that point by 2050. Among its clean energy policies, the Golden State has mandated that by 2020, the largest utilities make one-third of their electricity from green sources, a standard they are already meeting.

California also has a low-carbon fuel standard, which aims to grow the use of alternatives to gasoline and diesel. It sets a goal of lowering fuels' average carbon content 10 percent within six years.

Rob Oglesby, executive director of the California Energy Commission, rejected the claim that California's carbon reduction goals take precedence over water protection. He co-chairs the Water Energy Team of the Climate Action Team (WET CAT), a panel that has examined both the energy used to move water and the water consumed for energy, Oglesby said.

"It's kind of a false choice," Oglesby said. "You can have sound water policies and still meet your carbon goals. We're working really hard to coordinate both of the policy areas."

Is there planning coordination?

Terrie Prosper, spokeswoman for the California Public Utilities Commission, said state agencies consider water when planning to meet power needs. She pointed to a rule requiring many power plants to stop once-through cooling, where water is taken from the ocean or other nearby water body and passed through the plant.

"We have also denied permits for renewable facilities due to their water usage," Prosper added. She clarified later that the companies proposing the plants had withdrawn applications because it was clear they would be rejected. She did not respond to a question asking her to identify those companies.

Some who have studied the issue, however, say that there has traditionally been a lack of coordination between agencies in charge or water and those that oversee energy.

Agencies, regulators and policymakers "put together these energy portfolios without thinking about water," asserted Newsha Ajami, director of urban water policy for Water in the West at Stanford University. "They assumed there's water available to generate this energy."

"The way they have these two resources managed is that it's very disjointed," she added. "You have an agency that deals with energy planning and regulation. You have, like, four or five agencies that deal with water. But neither of them necessarily talks to each other. Even though they are very much connected, we don't manage them in a very integrated way."

A report from the California Public Utilities Commission issued in January 2013 said that "there has not been as much focus on water use by the energy sector in the context of the [water-energy nexus]. ... However this does not mean that water is not a concern -- in California water quality is the driving feature."

"The main challenge for the Nexus, however, is not merely accounting for water use but is one of assessing the value and impact of water in electricity generation," it added. "The choice of the technologies then comes down to the value of the energy saved vs. the value of the impact on the water resource."

The primary water used for electricity in California is salt water, which does not compete with the freshwater supply, the report said, meaning "the main tradeoff is simply cost, higher GHG emission rates per KWh of generation, and the environmental impacts to the sources and returned water."

A chart provided by the California Energy Commission showed that once-through cooling plants larger than 20 megawatts used more than 2 trillion gallons of ocean and estuary water annually. The bulk of that was at nuclear plants. One of the two nuclear facilities in California -- San Onofre in San Diego -- closed last June. It still uses water for cooling, but less than before. The chart cited data from 2010 and, to fill in gaps where necessary, 2009 and 2008.

Geothermal plants annually used 7,631 million gallons of fresh water and 13,708 million gallons of recycled water. Biomass plants in the state annually used nearly 3,424 million gallons of fresh water, and solar thermal plants used 841 million gallons of fresh water.

Shift to dry cooling

California Energy Commission officials said that they are requiring most new plants to use dry cooling, in which air is used for cooling instead of water.

"That's pretty much the starting point for new projects, is dry cooling," said Roger Johnson, director of the Energy Commission's Siting, Transmission and Environmental Protection Division. "The developer would have to justify why dry cooling wasn't the appropriate cooling technology for the project."

Dry cooling isn't as energy-efficient, Hightower said, and doesn't work well in hot climates. It often requires the use of big fans. In some places, 10 percent of the power load is used for running fans. That means greater cost and more greenhouse gas emissions, he said.

"It's a decision" between water or energy, Hightower said. "You have to trade it off."

Johnson, however, said that dry cooling towers have improved and can almost mimic the performance of wet cooling towers. Plants built recently in the California desert -- like the Ivanpah Solar Electric Generating System in the Mojave Desert and Phase 1 of the Genesis Solar Energy Center near Blythe -- are using dry cooling, he said. Ivanpah, which is the world's largest solar thermal plant, proposed dry cooling when it submitted plans to the California Energy Commission.

Ken Johnson, a spokesman for the Solar Energy Industries Association, said that "the choice of cooling technology is the real key here."

"Areas with water scarcity tend to favor water-efficient dry cooling over wet cooling," he said. "Dry cooling comes at a cost, but whether it's worth it is decided on a case-by-case basis. Some CSP facilities use dry cooling, as do some conventional generators."

In California, "water use also depends on site-specific considerations," said Roger Johnson with the California Energy Commission. "If a project owns water rights, it may use them within environmental constraints." Or, in the case of geothermal plants in the state's Imperial County, "if it has cheap water that it is willing to supply to users, plants are wet-cooled and appear to be large water users."

"In the case of power plants licensed by the Energy Commission, we have actively looked to reduce water use at the plants, regardless of technology or location," he said. In the future, "we will permit wet-cooled projects that use recycled water."

California's Air Resources Board, or ARB, which administers the state's low-carbon fuel standard, doesn't look at water from the depletion standpoint, said ARB spokesman Stanley Young.

"The GHGs associated with crop production (and hence water use) have been factored into the life cycle analysis that drives the carbon intensity of different fuels," Young said in an email. "Since our goal is [GHG] reduction we have not yet focused in the absolute water use element except as it relates to GHGs."

Biofuels that can meet the low-carbon fuel standard include ethanol from corn, sugar cane and biodiesel from soybeans. The special gasoline blend already required in California includes 10 percent ethanol.

"Most of the corn used in the [California] plants comes from the Midwest, so the percentage of water use for [California] ethanol is lower still," Young said. About 13 percent of ethanol is made in-state, ARB said. About 13 percent of ethanol used in the state is made in California, he said.

Some dispute the credibility of Hightower's data on biofuels.

Don Scott, director of sustainability at the National Biodiesel Board, said the numbers are from a 2006 report and that the industry has changed significantly in eight years. Tom O'Connor, a water expert who analyzed the issue for the Biodiesel Board, found that for crops grown in Iowa, less than 2 gallons of water is used to make a gallon of biodiesel.

State-specific analysis of water and biodiesel crops is better than using a natural average "because it allows you to understand more about how and why irrigation is used," Scott said.

Additionally, Scott said, "the biggest failing of the Sandia numbers is that it assigns all of the water use to biodiesel and none to the other products produced." In the case of soybeans, a large part of the crop normally is used for livestock feed.

"If you assign all of the water used to grow the crop to biodiesel, that would be like saying no water was used to produce the livestock feed," Scott said, adding, "The other big mistake in the Sandia math is that they assume all soybeans would be irrigated, when only about 10 percent of the acres are irrigated. So the national average water use is much less."

Hightower said that while his statistics are from 2005 for a report issued in 2006, the numbers haven't altered notably. Ajami with Stanford said Hightower's data are in line with the ones she views as accurate.

Correction: An earlier version of this story gave incorrect figures for the amount of water used in power plants larger than 20 megawatts, geothermal plants, biomass plants and solar thermal plants.

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